One of the causes of the error Could not create shared cache. (0,0) on the signon screen is if the application server is running low or has run out of disk space.

Large log files from traces are a common cause of this error, particularly if you are using application server tracing, so once you clear up some space, check that you don't have any unnecessary tracing turned on. Freeing up some disk space seems to resolve the error without the need for any server restarts.